1. Field of the Invention
The present invention relates to a dielectric ceramic composition, and more specifically, it relates to a dielectric ceramic composition capable of forming a dielectric substance which has a relatively high dielectric constant and excellent temperature properties thereof and which is suitable for a multilayer ceramic capacitor by firing the composition at a low temperature of 1100.degree. C. or less.
2. Description of the Related Art
As a dielectric ceramic composition for use in a multilayer ceramic capacitor, a composition containing barium titanate as a main component (hereinafter referred to as "the barium titanate compound") has been heretofore widely used. In particular, the barium titanate compound has been widely used as a dielectric ceramic composition for a multilayer ceramic capacitor having Y5V characteristics -30.degree. C. to +85.degree. C., .DELTA.C/C (20.degree. C.), +22% to -82%! and X7R characteristics -55.degree. C. to 125.degree. C., .DELTA.C/C (20.degree. C.), .+-.15%! of EIA standards.
A firing temperature of the barium titanate compound is as high as 1300.degree. C. or more, and therefore, as an internal electrode for the barium titanate compound, inexpensive silver palladium cannot be used because of a low melting point. An inexpensive base metal such as nickel has a high melting point, but when it is baked in air, its oxide is formed inconveniently. For this reason, the base metal cannot be used, either. Accordingly, it is necessary that expensive platinum, palladium or the like should be used as the internal electrode, which leads to the increase of cost. Furthermore, the barium titanate compounds having temperature properties (e.g., Y5U characteristics or Y5T characteristics) between the Y5V characteristics and the X7R characteristics have scarcely been reported.
On the other hand, in order to solve problems in the case that the barium titanate compound is used as a dielectric substance for the laminated ceramic capacitor, the application of a lead-containing perovskite compound to the dielectric substance has been widely researched, and a part of the researches has been put to practical use.
Some of lead-containing perovskite compounds have a firing temperature of 1100.degree. C. or less, and when any of such compounds is used, there is an advantage that inexpensive silver palladium or the like can be used as an internal electrode for the multilayer ceramic capacitor. In addition, by combining a plurality of lead-containing perovskite compounds, there can be widely designed various materials ranging from a high dielectric material having a dielectric constant of 2000 or more at room temperature to a material which scarcely changes at a temperature of such a dielectric constant as to meet the X7R characteristics of the EIA standards. Examples of the former include three kinds of lead-containing perovskite compounds such as lead magnesium niobate PMN: Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 !, lead titanate (PT: PbTiO.sub.3) and lead nickel niobate PNN: Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 ! mentioned in Japanese Patent Application Laid-open No. 81097/1973. Examples of the latter are mentioned in, for example, Japanese Patent Application Laid-open No. 060671/1983, 042277/1985 and 036371/1985.
However, in Japanese Patent Application Laid-open No. 060671/1983, it is described that a capacitance change of the capacitor by a temperature at 30.degree. to 85.degree. C. can be controlled to .+-.22% or less (which corresponds to the Y5S characteristics of the EIA standards) by the use of the lead-containing perovskite compound as the dielectric substance, but there is a problem that the dielectric constant at room temperature lowers in most of the cases that compositions capable of decreasing the change of the capacitance by the temperature are employed.
Furthermore, there also exist a means for improving the temperature properties of the capacitance by adding a manganese compound, but in this case, there is a problem that a second phase of a pyrochlore phase, lead oxide or the like is formed, so that the dielectric constant noticeably lowers.
As another means for improving the temperature properties of the capacitance, a method has been present which comprises replacing a part of lead (Pb) in the lead-containing perovskite compound with barium (Ba), strontium (Sr) or calcium (Ca) which is another alkali earth metal. For example, Japanese Patent Application Laid-open No. 298061/1989 has disclosed a composition in which a part of Pb in a complex perovskite compound of lead magnesium niobate PMN: Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 !, lead titanate (PT) and lead zirconate (PZ: PbZrO.sub.3) is replaced with at least one element of Ba and Sr. However, some of the complex perovskite compounds mentioned in Japanese Patent Application Laid-open No. 298061/1989 have a firing temperature of 1000.degree. to 1300.degree. C., so that for a part of such complex perovskite compounds, inexpensive silver palladium cannot be utilized as an internal electrode.
Furthermore, Japanese Patent Application Laid-open No. 115408/1992 has disclosed compositions in which a part of Pb in a composite of lead magnesium niobate (PMN) and another lead-containing perovskite compound is replaced with Ba, Sr, Ca or Ag. In these compositions, however, it has been difficult to control the temperature properties of the dielectric constant to .+-.22% or less in the range of -30.degree. to +85.degree. C.
On the other hand, in Japanese Patent Application Laid-open No. 223162/1991, there have been disclosed compositions in which lead manganese niobate PMnNb: Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3 ! is added to a complex perovskite compound of lead magnesium tungstate PMW: Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 !, lead nickel niobate (PNN), lead titanate (PT) and lead zirconate (PZ). In these compositions, the dielectric constant at room temperature is 8000 or more and the Y5T characteristics regarding the capacitance (-30.degree. C. to +85.degree. C., .DELTA.C/C (20.degree. C.), +22% to -33%) can be met, but there is still a problem that a change rate of the dielectric constant cannot be controlled to .+-.22% or less.